Pretreating process for vitreous enamelling

ABSTRACT

A pretreating process of steel sheet surface for vitreous enamelling, by roughening the surface by electrolysis and covering the roughened surface with a water glass layer, the roughened surface may be metal plated and a wax cover layer may be formed on the water glass layer.

United States Patent 1 Harada et a1.

[52] US. Cl. 29/424; 29/527.4; 72/46; 204/38 C; 427/156; 427/330 [51] Int. Cl. B231 17/00; 8220 11/126 [58] Field 01 Search 204/129.1, 206, 38 C; 117/213, 219, 53, 70 C; 29/424, 527.4

[56] References Cited UNITED STATES PATENTS 12/1919 Bognar 117/219 11/1937 Kautz 117/53 X 1 1] 3,927,460 [451" Dec. 23, 1975 2,384,542 9/1945 Fruth et a1. 1 17/213 2,807,558 9/1957 Pankove 117/213 3,258,947 7/1966 Wehmeyer 72/42 3,471,375 10/1969 Cooke et a1. 204/28 3,650,935 3/1972 Andersson 204/206 3,706,124 12/1972 Leontaritis et a1 29/424 3,753,870 8/1973 Hoffman et a1 204/34 FOREIGN PATENTS OR APPLICATIONS 566,306 12/1944 United Kingdom l48/6.15

Primary Examiner-F. C. Edmundson Attorney, Agent, or Firm-Fleit & Jacobson [57] ABSTRACT A pretreating process of steel sheet surface for vitreous enamelling, by toughening the surface by electrolysis and covering the roughened surface with a water glass layer, the roughened surface may be metal plated and a wax cover layer may be formed on the water glass layer.

10 Claims, 16 Drawing Figures US. Patent Dec. 23, 1975 Sheet 1 of6 3,927,460

US. Patent Dec. 23, 1975 Sheet 2 of 6 F i g 3 R0 Era/ling Time I I v n sp current A n a 308 lzzolmlesolzaa 230 250 270 Pack mun? U.S. Patent Dec. 23, 1975 Sheet 3 of6 3,927,460

FIG.4A FIG.4B

FIG.4C

U.S. Patent Dec.23, 1975 Sheet4of6 3,927,460

FIG.5C

US. Patent Dec. 23, 1975 Sheet 5 of 6 3,927,460

/7 4% 2 6 25' 22 Fig.7

US. Patent Dec. 23, 1975 Sheet 6 of6 3,927,460

FIG. 8A FIG. 8B

FIG.OC

PRETREATING PROCESS FOR VITREOUS ENAMELLING This invention relates to a pretreating process for vitreous enamelling, and more particularly to a process of pretreating steel sheet, plate, or article for vitreous enamelling so as to ensure steadfast adherence of vitreous enamel coating to the steel sheet, plate, or article.

To pretreat steel workpiece, e.g., sheet, plate, or article, for vitreous enamelling, it has been practised to apply liquid or paste-like oil coating on the surface thereof for purposes of protecting the surface from scratching in the course of shaping by press or the like. Such oil coating, however, cannot completely eliminate scratches on the surface of the workpiece, which are caused by abrasion with dies, because the liquid or paste-like coating tends to be peeled off from the workpiece surface at locations where high pressure is exerted from the dies to the workpiece.

To mitigate the risk of localized scratching, the socalled strippable coating, which uses a resin film, or the so-called washable coating, which is removable by washing with water or a suitable organic solvent, has been proposed recently. Both the strippable coating and the washable coating use a solid or nearly solid lubricant layer, which lubricant layer remains on the workpiece surface even when localized high pressure is applied to it, so that the risk of scratching of workpieces and flaws on the die surface can be removed.

The strippable coating is, however, costly because a vinyl film or a fluorine-contained resin film must be bonded to the surface of each workpiece by a suitable adhesive. Furthennore, the vinyl or fluorine-contained resin film must be removed from the workpiece after the shaping through a time-consuming laborious process. Due to such shortcomings, the user of the strippable coating has been restricted only to special purposes.

With the washable coating, water-soluble material is used. If metallic soap or a water-soluble polymer is used to make the washable coating, such material does not have any water-repellency, so that one cannot expect any corrosion resistance in the washable coating formed thereby. On the other hand, if wax type material which is soluble in an organic solvent is used to make the washable coating, it is difficult to find a practical means for applying the wax type material with a uniform thickness. Furthermore, an extra amount of the organic solvent is necessary for removing the wax type material film from workpieces.

An object of the present invention is to obviate the aforesaid difficulties of the conventional pretreating processes of steel workpiece for vitreous enamelling, by providing an improved pretreating process comprising a step of applying a water glass layer on the roughened surface of a workpiece, i.e., steel sheet, plate, or article, which is to be coated with vitreous enamel. The workpiece surface is roughened prior to the application of the water glass layer. With such water glass layer, the workpiece is fully protected from scratching in the shaping process, e.g., by press, prior to the vitreous enamelling.

The inventors have found out that if a 2 to 5 micron thick water glass layer is provided on the surface of steel workpiece and a liquid lubricant, e. g., press oil, is spread on the water glass layer, the deep drawability of the workpiece is greatly improved, and the risk of 2 scratching the workpiece is greatly reduced in the shaping process.

The mechanism of the elimination of the scratches by the extremely thin water glass layer on the workpiece seems to be as follows. The thin water glass layer is so strong that it can withstand a very high localized pressure, which is caused by small undulations inherent to the workpiece surface or undulations newly formed in the course of shaping. Thus, the thin water glass layer remains between the workpiece and an upper shaping die, and between the workpiece and a lower shaping die or a supporting table. The presence of such thin water glass layer between the workpiece and the acting parts of the shaping machine acts to disperse any local concentration of pressure to surrounding areas, so as to nearly completely eliminate the scratching of the workpiece surface caused thereby. When the shaping die or ram forcibly slide along the workpiece surface, the thin water glass layer may be scraped by the die or ram, but the workpiece surface itself is protected by the scraping of the water glass layer. On the other hand, a conventional soft fluid film, such as the lubricant or press oil film, is too weak to withstand the localized high pressure which is built up on the workpiece surface. Accordingly, the soft fluid film is broken in the course of the shaping process, and the workpiece surface is exposed to direct contact with the shaping die or ram, and the workpiece surface is scratched as the workpiece moves relative to the shaping dies.

Water glass is relatively inexpensive, and it is widely used in various industries. It is known that the aqueous solution of water glass can be used as a degreasing agent. Accordingly, an oil film applied on the water glass layer can be removed more easily than that applied directly on the workpiece surface. As pointed out in the foregoing, the conventional washable coating, which is made of an aqueous solution of metallic soap or water-soluble polymer, lacks water repellency and corrosion resistance. The water glass layer is easily soluble in water, yet it ensures a high corrosion resistance because the aqueous solution of water glass has a strong alkalinity which inactivates the workpiece surface.

For practical application of the water glass to a steel workpiece, e.g., sheet, plate, or article, it is preferably to cover the surface of the water glass film with a moisture-proof layer, so as to prevent excessive absorption of moisture by the water glass. The excessive moisture absorption by the water glass tends to cause the efflorescence of the water glass itself into whitish powder particles. Once the water glass changes into the whitish powder, it loses the corrosion resistance. The moisture absorption by the water glass also tends to make it more viscous. Accordingly, if it is necessary to stock water glass coated steel sheets as piled for an extended period of time, the excessive moisture absorption by the water glass tends to cause the steel sheets to stick with each other.

As a result of search for the material of the aforesaid moisture-proof cover of the water glass, the inventors have found out that the desired water-proofness can be achieved by covering the water glass with a layer of wax which is in solid state at room temperature. Thus, provision of such wax layer on the water glass layer does not deteriorate the aforesaid improvement of the scratch prevention. In cases which require the storage of water glass coated workpieces for a long time, it is desirable to cover the outer surface of the water glass layer with the aforesaidwax which is in solid phase at room temperature. The water glass layer on the workpiece surface may be dried before covering it with the aforesaid wax.

In an embodiment of the present invention, an aqueous solution of water glass, consisting of four parts by volume of water and ten parts by volume of water glass, is applied to the surface of a steel sheet by a conventional roller coating, and it is dried by blowing hot air thereto. After being dried, the water glass coated steel sheet is dipped in a wax solution, which is prepared by dissolving normally solid wax in toluene at a rate of 100 grams of the wax in I liter of toluene. Instead of dripping in the wax solution, dipping in molten wax orroll-coating may be used for covering the water glass coating with a wax layer. The thickness of the water glass layer to be applied to the steel sheet is preferably 2 to 5 microns, and the solid wax layer overlaid on the water glass layer should preferably be 0.5 to 1 micron thick. With the water glass layer and the wax cover of the aforesaid thicknesses, an excellent deep drawability was achieved. Even after deep drawing, the steel sheet can be coated with a vitreous enamel layer which is strongly adhered thereto.

To ensure strong adherence between the steel sheet and the vitreous enamel layer, the surface of the steel workpiece or sheet is roughened prior to the application of the water glass.

Another object of the present invention is to provide a pretreating process for vitreous enamelling, which includes an improved roughening of the workpiece surface, e.g., steel sheet, prior to the application of water glass thereto.

In conventional vitreous enamelling processes, workpiece, e.g., steel sheet, is shaped, cleansed, and then etched by dipping the workpiece in an aqueous solution of sulfuric acid or nitric acid. The etching is to roughen the workpiece surface, and vitreous enamelling may be effected on the workpiece thus etched. An extremely thin coating of nickel, cobalt, molybdenum, or phos phate, may be applied to the etched surface of the workpiece prior to the vitreous enamelling, for improving the bondage of the enamel with the workpiece. A typical example of such known pretreatment for the vitreous enamelling is disclosed in British Patent specification 763,379, which was granted to Gerald Gordon et al and published on Dec. 12, I956. It is also possible to apply chromate treatment to the roughened workpiece surface. Another object of the present invention is to provide an improved etching method of workpieces, especially steel sheets, prior to applying vitreous enamelling thereto. With the pretreatment of steel sheets or workpiece by the etching method according to the present invention, each steel is electrolytically etched by using liquid electrolyte which is forced to move rapidly during the etching operation, so that a plurality of pyramid-shaped deep pits are etched on the surface of the sheet or workpiece.

Thus, the etching operation of the present invention can be carried out while spreading steel sheet flat, and the etched sheet is shaped after the etching, e.g., by press, and then the vitreous enamelling is applied to the shaped sheet. whereby, the overall efficiency of the vitreous enamelling process inclusive of the etching can greatly be improved. With the etching operation of the invention, a liquid electrolyte is injected to the flatly spread steel sheet surface, and hence, the steel sheet can be handled more easily than in the case of etching by dipping the steel sheet in an electrolyte cell.

With conventional etching prior to enamelling, a steel sheet or plate is anodized in an electrolyte cell by moving the sheet or plate relatively slowly in the electrolyte. In this case, the steel sheet or plate acts as an anode, and hydrogen gas tends to be trapped on the anode surface and the entrapped hydrogen interferes with the etching so as to weaken the adherence of vitreous enamel with the steel sheet or plate. With the conventional etching, if one tries to move the steel sheet at a comparatively high speed, the size of the electrolyte cell must be expanded, because the steel must be exposed to the electrolyte for a given length of time for achieving the desired etching effect. After years of studies, the inventors have found out that the aforesaid difficulties of conventional etching pretreatment for vitreous enamelling can be obviated by using a comparatively high relative speed between the liquid electrolyte and the steel sheet. Furthermore, the high relative speed between the liquid electrolyte and the steel sheet also roughens the steel sheet surface so as to form acutely angled projections and recesses which are densely distributed on the surface of the steel sheet. Thus, the inventors have succeeded in greatly strengthening the bondage of the vitreous enamel layer and the steel sheet and in improving the electrolytic etching operation without causing any expansion of the etching facilities, by providing the aforesaid etching which uses a comparatively high relative speed between the liquid electrolyte and the steel sheet.

For a better understanding of the invention, reference is made to the accompanying drawing, in which:

FIG. 1A is a fragmentary sectional view of a steel sheet, which is provided with a water glass layer as the steel sheet is rolled;

FIG. 1B is a fragmentary sectional view of a water glass coated steel sheet, which is pretreated by a process of the invention prior to vitreous enamelling;

FIG. 1C is a fragmentary sectional view of a steel sheet which is coated with a phosphate layer and a water glass layer by a process according to the present invention;

FIG. 2 is a schematic diagram of an electric circuit for testing an etching process by using an electrolyte cell having a rotary drum;

FIG. 3 is a graph, illustrating the relation between the peak count and the PEI (Porcelain Enamelling Institute) index (percent);

FIGS. 4A and 5A are pictures of the surface conditions of steel sheets etched by using a stationary electrolytic cell and a rotary electrolytic cell, respectively, which pictures are taken by a scanning electron microscope with a magnification of 1,000;

FIGS. 48 and 5B are pictures similar to those of FIGS. 4A and 5A, respectively, except that the magnification is increased to 3,000;

FIGS. 4C and 5C are pictures similar to those of FIGS. 4A and SA, respectively, except that the magnification is increased to 10,000;

FIGS. 6 and 7 are a side view and a plan view of a device suitable for carrying out the process according to the present invention, respectively; and

FIGS. 8A, 8B, and 8C are pictures of the surface conditions of a steel sheet etched by using a rapidly moving liquid electrolyte in the device of FIGS. 6 and 7, which pictures are taken by using a scanning electron microscope, with magnifications of L000, 3,000

and 10,000, respectively. FIG. 1A shows a fragmentary view of steel sheet A which is pretreated by a process according to the present invention. Referring to the figure, a dry water glass layer B is provided on the rough surface of the steel sheet A, and a lubricant layer C covers the top surface of the water glass layer B.

FIG. 1B shows a steel sheet A1, which is etched and provided with a nickel layer E deposited thereon prior to forming a dry water glass layer B thereon. A lubricant layer C is overlaid on the water glass layer B.

FIG. 1C illustrates a steel sheet Al, which is provided with a phosphate layer D. A lubricant layer C is overlaid on the phosphate layer D.

When a steel sheet as rolled is shaped, for instance, by press, prior to vitreous enamelling, the undulations of the sheet surface tend to be flattened or levelled off, while possibly causing scratches on the levelled surface. Such flattening of the undulations and the risk of scratches on the flattened surface are inevitable, even if the steel sheet surface is coated with nickel or phosphate. The flattened surface and scratches tend to weaken the adherence between the steel sheet and the vitreous enamelling layer thereon, and also to deteriorate the appearance of the enamelled goods. Especially, the phosphate coated steel sheet must be carefully handled during the shaping, so as not to peel off the phosphate layer. The phosphate layer itself acts as a lubricant during press work. A suitable soap material may be used together with the phosphate layer. This phosphate layer should preferably be kept on the steel sheet surface even after the shaping by the press, so as to ensure strong adherence of paint coating or vitreous enamel coating to the steel sheet. If, however, the phosphate layer is scratched or adherence during the shaping by press, its function is greatly deteriorated.

The inventors have found out that, if a very thin layer is comparatively less flexible dry water glass is applied to the steel sheet surface, the roughened conditions of the steel sheet surface can be maintained all through the shaping process by press.

The reason for such improvement seems to be as follows: namely, if all the minute recesses in the entire rough surface of the steel sheet is covered by the comparatively hard material, i.e., water glass, while filling up the recesses with it, as shown in FIGS. 1A, 1B, and 1C, and the glass water layer bears local concentration of high stress during the shaping so as to prevent the deformation of the rough surface conditions, e.g., removal of projections and filling of recessed portions with steel from the projections.

It was also found that, with the pretreatment of the invention, steel sheets having smoothed surfaces, e.g., bright finished steel sheets, can be shaped by press without causing any scratch on the surfaces thereof. The comparatively hard water glass is applied to the steel sheet surfaces at such thickness that the water glass layer itself may be scratched by moving parts of a shaping machine but the steel sheet surface under the water glass can be kept intact.

The application of water glass to the steel sheet prior to vitreous enamelling will now be described in further detail by referring to examples.

EXAMPLE 1 Two specimens of steel sheets to be protected by the process according to the present invention were prepared by using cold rolled steel sheets of 0.8 mm thickness (finished by Dull Finish 3.0 p. l-lmax), which were 6 coated with dry water glass layers of 5 microns and 3.5 microns thickness, respectively, and top cover layers consisting of a suitable press lubricant were overlaid on the water glass layers.

Four test specimens without any water glass coating were prepared by using the same steel sheets of 0.8 mm thickness while applying cover layers of lubricant.

All the specimens were drawn into the shape of cups in substantially identical fashion, and the formation of scratches and flattening effects of different specimens were compared. Each specimen cup was 55 mm deep, which was made by punching a blank with a diameter of 200 mm by a flat bottom punch of mm diameter while using blank-holder load of 5 tons. The outer surface of the drawn portion was exposed to scratching and flattening. The glossiness of the outer surface increases as the number of scratches and the degree of flattening increase.

The glossiness of the outer surface of the specimen cups was measured by the method as stipulated in JIS Z8741 and ASTM D523. The results are shown in Table 1. Small values of the glossiness (percent) in Table 1 represent good protection of the rough specimen surfaces by the protective layers; namely, roughly speaking, the smaller the glossiness is, the better the protection is.

Table l Gloss Gs (0") (3 locations for each specimen) plus press lubricant Gloss Gs (0) according to H5 Z874l As apparent from Table 1, the gloss of the specimen cup outer surface is reduced to about one half by adding a water glass layer on steel sheet, as compared with that of specimen cups with the press lubricant alone. With the water glass layers of 5 microns and 3.5 microns thickness, there was substantially no scratches and flattened portions produced by the shaping, which can be detected by sight. Thus, with such protective layers, the surface conditions of the steel sheet prior to the press shaping are maintained even after the steel sheet is shaped by press.

On the other hand, with the press lubricant layer alone, a considerably large number of scratch lines and flattened portions were produced on the steel sheet surface by the shaping.

A typical example of the improvement of the deep drawability was as follows. When 1.6 mm thick SPCC (JlS) class steel sheet was drawn by press while applying a 5 micron thick water glass layer plus a press lubricant (Press Oil No. 620 made by NIHON KOSAKUYU KABUSHIKI KAISHA) layer thereon, a limiting drawing ratio L.D.R. of 2.25 was obtained. On the other hand, when the same steel sheet was drawn by a similar manner without using any water glass layer, its limiting drawing ratio L.D.R. was only 2.05. lt is generally believed that high viscosity of press lubricant gives favorable effect on the deep drawability, and the water glass layer or the like solid phase layer acts to provide a very 7 high viscosity.

EXAMPLE 2 Two specimens of steel sheets to be pretreated by the process according to the present invention were prepared by using cold rolled steel sheets of 1.8 mm thickness (bright finished), which were coated with dry water glass layers of 5 microns and 3.5 microns thickness, respectively, and top cover layers consisting of a suitable press lubricant.

One test specimen without any water glass coating was prepared by using the same steel sheet of 1.8 mm thickness while applying a cover layer of lubricant.

All the specimens were drawn in substantially identical fashion, and the formation of scratches and flattening effects of different specimens were compared. Each specimen was drawn to a depth of 85 mm by using a flat bottom punch of 140 mm diameter, a blank with a diameter of 280 mm, and a blank-holder load of 6 tons.

The specimen, which was protected only by the press lubricant layer, produced innumerable parallel scratches on its outer surface upon the aforesaid drawing. The parallel scratches were greatly reduced when the specimen was provided with a 3 micron thick water glass layer. With the 5 micron thick water glass layer, the generation of the scratches by the drawing was almost completely eliminated.

The degree of glossiness of the specimens was measured in the same manner as Example 1. The results are shown in Table 2.

As apparent from Table 2, the specimen with press lubricant alone had the largest degree of glossiness, not because of the bright finishing prior to the drawing, but because of the generation of scratches in the course of drawing.

[t was found that the degree of glossiness of the specimens with water glass layers was reduced despite the lack of the generation of scratches. The reason for this reduction of the glossiness is the fact that fine undulations are caused on the steel sheet surface by the slipping and rotation of the crystal lattice of the steel sheet during the plastic deformation in the shaping operation, so that the initial surface glossiness of the steel sheet cannot be maintained. The merit of using the water glass layer, however, is not affected by such change of the glossiness.

EXAMPLE 3 Two specimens of steel sheets to be pretreated by the process according to the present invention were prepared by applying dry water glass layers of 5 microns and 3.5 microns thickness to phosphate coated steel sheets, respectively, and applying top cover layers consisting of a suitable press lubricant.

One test specimen without any water glass coating was prepared by using the same phosphate coated steel sheet by applying a soap layer thereon.

The specimens thus prepared were subjected to drawing in the same manner as Example 1, and the results are shown in Table 3.

Table 3 Gloss Gs (0) (3 locations for each specimen) Surface layers Location Mean Soap alonc 27.76 38.40 34.40 33.52 3.5;; thick water glass 32.16 27.20 30.40 29.92 plus press lubricant 5 thick water glass 26.40 29.60 29.04 28.32

plus press lubricant EXAMPLE 4 Four specimens of extra-low-carbon steel sheets for vitreous enamelling were processed by applying sulfuric acid etching (until a weight reduction of 2 g/ft was achieved), and nickel plating them (at a rate of 50 mg/ft Three specimens to be protreated by the process according to the present invention were prepared by using the extra-low-carbon steel sheets thus processed by applying dry water glass layers of 3.5 microns, 2.5 microns and 1.5 micron thickness thereon, respectively.

One test specimen without any water glass coating was formed by using the extra-low-carbon steel sheet as processed in the foregoing manner.

All the specimens thus prepared were subjected to drawing tests, in the same manner as Example 1. The results are shown in Table 4.

Table 4 Thickness of water glass layer Bonding strength (micron) of vitreous enamel 0 Poor 1.5 Poor 2.5 Fair 3.5 Excellent As can be seen from Table 4, with the 3.5 micron thick water glass layer, the etched surface of the extralow-carbon steel sheet with nickel plating can be protected almost completely, so that a very strong bondage is ensured between the steel sheet and the vitreous enamel layer.

Generally speaking, the surface conditions of steel sheet is very important for providing good vitreous enamelling layer thereon. 1f the steel surface is completely flattened, it is very difficult to provide strong bondage between the steel sheet and the vitreous enamelling layer.

The advantage of applying a water glass layer on a steel sheet surface for purposes of pretreating the steel sheet for vitreous enamelling are as follows.

a. The steel sheet can easily be degreased after shaping, e.g., by press, because the water glass itself has a degreasing ability. Upon completion of shaping the steel sheet into a desired form, the water glass is removed by washing the steel sheet with cold water, hot water, or an aqueous solution of a suitable degreasing agent. Lubricants and waxes, which are applied to the steel sheet for facilitating the shaping operation, can be removed together with the water glass by the aforesaid washing operation for the removal of water glass.

Table shows the difi'erence of degreasing speeds for removing press lubricant between a steel sheet with a 3 micron thick water glas layer and another steel sheet without any water glass layer. For the degreasing, the steel sheets were dipped in a 2 percent aqueous solution of caustic soda at 80C.

Table 5 Extent of degrease (after washing for the following periods, in second) It) 30 60 Surface condition 2 5 I 300 600 ND ND HD HD D CD CD CD ND: not dcgreascd HD: halfway degrcased I): almost degreased CD: completely degreased b. The time required for drying the water glass layer is much shorter than that for drying a conventional organic substance coating. Thus, the formation of the water glass coating is easier than conventional coatings. The water glass coating according to the present invention is free from the risk of aging of steel sheet which is caused by baking.

c. The water glass is an inexpensive material, so that the entire pretreating process can be carried out at a low cost.

d. The water glass is a material which is similar to cover coat for vitreous enamelling, so that any residue of the water glass will cause no adverse effects to the vitreous enamelling layer.

It should be noted here that, although the application of the water glass layer has been described as a part of pretreating process for vitreous enamelling, the water glass itself is also useful for protecting bare or metalplated surface of a steel sheet for shaping in general.

More practical examples of the present invention will now be described.

EXAMPLE 5 Water glass was applied to the surfaces of a 0.8 mm thick steel sheet for vitreous enamelling, so as to form a 3.5 micron thick dry water glass layer thereon. A press lubricant layer was formed on the water glass layer, and the steel sheet thus protected was drawn into the shape of a cooking pot. After degreasing and washing with water, a l30 micron thick white vitreous enamel layer was applied and baked thereon in one operation.

The vitreous enamel layer of the cooking pot thus made was found to be flawless, and the vitreous enamel layer was very strongly bonded to the steel sheet of the cooking pot.

EXAMPLE 6 Water glass was applied to the surfaces of a phosphate-coated steel sheet, so as to form a 3.5 micron thick dry water glass layer thereon. A press lubricant layer was formed on the water glass layer, and the steel sheet thus protected was subjected to working by press. It was provided that the phosphate coating was well protected and not scratched during the press working. Then, the steel sheet thus worked was painted, and the paint proved to have a very strong bondage to the steel sheet. The steel sheet thus painted had a very high corrosion-resistance.

EXAMPLE 7 Water glass was applied to the surface of [.8 mm thick steel sheets, so as to form a 5 micron thick dry water glass layer on each of the steel sheets. A press lubricant layer was formed on the water glass layer, and the steel sheets thus protected were shaped by press. No scratches were formed on the steel sheet surface. After smoothing the roughened portions of the steel sheets by light buffing, lustrous platings of copper, nickel, and chromium were applied to them, respectively. The results proved to be satisfactory.

EXAMPLE 8 An aqueous solution of water glass consisting of four parts by volume of water and ten parts by volume of water glass was applied to first specimens of cold rolled sheets of rimmed steel by a roll coater, so as to form an about 3 micron thick water glass layer thereon after drying it at C for 30 seconds. Each of the water glass coated steel sheets was dipped in a wax solution, which was made by dissolving l00 grams of wax in 1 liter of toluene, so that a wax layer of about I micron thickness was formed thereon. The solvent for the wax may be benzene or any other suitable solvent. After applying the water glass and wax coatings, each steel sheet of the first specimens was shaped into a cup by press.

Separately, second specimens of cold rolled sheet, which were made of the same lot of the rimmed steel as the first cold rolled sheet, were coated with a high-class liquid lubricant alone and similarly shaped by press.

The workability of the first and the second sheets was tested, and the results are shown in Table 6.

Table 6 Item First specimen Second Remark (pretreated by specimen process of the (Conventionally invention) pretreated) 3-micron thick liquid lubricant water glass, plus coating, Press l-micron thick Oil No. 660

wax (made by NIHON KOSAKUYU KABUSHIKI KAISHA) Limiting 2.27 2.17 drawing ratio (L.D.R.) Flat-head Punch force at 2.90 tons 3. l 5 tons punch of 30 draw ratio of mm dia. 2. l7 Scratches on None Scratches on Based on I00 steel sheet outer surface specimens surface of shaped cup which were Clinging of None Die surface was continuously coating to die wettcd by pressed lubricant Corrosion test in No corrosion Heavily About l.5% SO atmosphere after l0 days corroded in 2 50,, relative after press clays humidity shaping greater than Table 6-continued ltem First specimen Second Remark (pretreated by specimen process of the (Conventionally in vention pretreated 3-micron thick liquid lubricant water glass, plus coating, Press l-micron thick Oil No. 660

wax (made by NlHON KOSAKUYU KABUSHIKI KAISHA) 95% Removability of Completely Removed in I20 Not agitated coating after removed in 30 seconds by press shaping seconds by dipping in a 5% dipping in warm solution of water at 60C degreasing agent (Homezarin F-lSO, made by Kawo Sekken Company) Sticking of No sticking in 30 Liquid lubricant Loaded at 100 specimens when days caused Kg per stored for a long hardly-separable 200mmX200- time sticking of steel mm As apparent from Table 6, the double coating according to the present invention has the following advantages.

a. The wax coating operation is simple. Since water glass layer is coated for the prevention of scratches, only a very thin wax coating is sufficient for corrosion prevention and lubrication, which wax coating can be applied by using a wax solution in a suitable organic solvent or by hot melt coating.

b. Excellent press workability can be ensured by the combined effect of the scratch prevention by water glass layer and lubrication by the wax coating.

c. The pretreated steel sheet can be stored for a comparatively long time, because the wax coating protect the water glass layer from the atmosphere, so as to prevent the water glass from reactions with air for eliminating the corrosion of the steel sheet.

d. The pretreated steel sheet can be stored even after the press shaping, because the wax coating protects it against moisture in the air. At portions where the wax coating is removed by the press shaping, the remaining water glass layer acts to passivate the steel sheet by the alkalinity of the water glass.

. The coatings can easily be removed by dipping the shaped sheet in warm water at a temperature higher than the melting point of the wax. As the wax melts, the water glass under the wax coating quickly dissolves in the warm water. Thus, the two coatings can very easily be removed from the shaped steel sheet.

f. Sticking of the steel sheets can be prevented by forming the wax coating with a wax having a melting point higher than about 50C.

g. No extra lubricant is necessary in the press shaping operation, as long as the pretreatment of the present invention is applied.

The inventors have found that the bondage between a steel sheet and a vitreous enamel layer applied thereto can be improved by roughening the surface of the steel sheet in an electrolyte which moves quickly relative to the steel sheet. The pretreatment including such roughening process will be described by other examples.

EXAMPLE 9 FIG. 2 illustrates the principles of surface roughening to be incorporated in the pretreating process according to the present invention. ln the figure, the running conditions of a variable-speed motor 1 are controlled by a regulator 2. Output power from the motor 1 is transmitted to a rotary shaft 4 through a suitable transmission, e. g., pulleys and a belt 3. One end of the rotary shaft 4 is secured to a rotary drum 5 which is concentrically disposed in a cylindrical electrolytic cell 7. A cylindrical cathode 6 made of lead is disposed between the rotary drum 5 and the inner surface of the side wall of the electrolytic cell 7. The rotary shaft 4 engages a brush 8, which is electrically connected to the positive potential terminal of a rectifier 9. The cathode cylinder 6 is connected to a negative potential terminal of the rectifier 9. The rotary drum 5 is selectively movable relative to the electrolytic cell 7, so that the drum 5 may be loaded in the cell 7 for processing and removed from the cell 7 for mounting or dismounting a steel sheet 10 thereto or therefrom.

An etching solution or liquid electrolyte, which consists of a dilute sulfuric acid and ferrous sulfate, is poured in the cell 7, so that the steel sheet 10 is dipped in the etching solution for causing an electric current to flow between the steel sheet 10 and the cathode 6. To electrically connect the steel sheet 10 to the rotary shaft 4, suitable conductor rings (not shown) are used, which are electrically connected to the rotary shaft 4 and mechanically adapted to hold the steel sheet 10 on the peripheral surface of the rotary drum 5. Thus, the steel sheet 10 acts as a rotary anode in the electrolytic cell 7, and the electric current between the anode and the cathode 6 acts on the surface of the steel sheet 10.

Tests were made by using 0.8 mm thick cold rolled low-carbon steel sheets of 50 mm width and 325 mm length. Referring to FIG. 2, a 30 mm spacing was provided between the steel sheet 10 and the cathode 6, as measured in the radial direction of the cylindrical electrolytic cell 7.

The etching solution used in the tests contained ferrous sulfate (H SO .7l-l O) at a concentration of 100 grams/liter and sulfuric acid (H at three different concentrations, i.e., l0 grams/liter, 20 grams/liter, and S0 grams/liter. The etching solution was kept at 80C, and its concentration was continuously monitored and maintained within a 10.] percent range of the desired concentration.

The current condition 30 A/dm X 30 sec. was used for each etching solution. The revolving speed of the rotary shaft 4 and the steel sheet 10 was selected to be 400 rpm, and tests were also made without rotating the shaft 4 and the steel sheet 10 for purposes of comparison. After being treated in the cell 7, the steel sheet 10 were nickel plated under different plating conditions. Then, a vitreous enamel layer was formed on the nickel plated steel sheet.

For each of the steel sheets which were pretreated under the aforesaid different conditions, the number of etching peak counts and the PEI (Porcelain Enamelling Institute) index were determined. The results are shown in Table 6. The etching peak count was measured by using a peak counter made by Bendix Company of the U.S.A., and the peak count in Table 7 represents the number of peaks, whose height is greater than 0.5 micron, per 25.4 mm line section on the steel sheet surface. The PEI index (percent), which repre- 13 sents the strength of the bondage of a steel sheet and a vitreous enamel layer formed thereon, was determined by a method stipulated both in Japanese Industrial Standard JIS R-4204 and in ASTM C-3l3-59. A PEI index 100 percent represents perfect bondage of a vitreous enamel layer to a steel sheet.

The nickel plating was applied after the etching of the steel sheet, in accordance with a conventional practice in vitreous enamelling. The nickel plating is to produce iron oxide (FeO) layer which assists, as an intermediate layer, the bondage between the steel sheet and the vitreous enamel layer. Four different nickel plating conditions were used, together with Watt bath.

Based on the data of Table 7, the relation between and C show similar pictures of a steel sheet which was etched by using it as a rotary anode, the magnifications of the pictures being 1,000, 3,000, and l0,000, respectively. In the case of rotary anode (FIGS. 5A to SC), square recesses or the so-called etch pits are formed with multiple-stepped side walls. On the other hand, with the stationary anode (FIGS. 4A to 4C), recesses formed by the etching are of simple valley shape without clearly defined stepped wall portions. Thus, the steel sheets etched by using them as rotary anodes have innumerable small but deep etch pits which may not be counted by conventional peak counters.

EXAMPLE the peak count and the PEI index is plotted in the graph The inventors further studied the effects of the reof FIG. 3, for different concentrations of sulfuric acid volving speed of the steel sheet acting as the anode by and different nickel plating conditions. In the figure, using a process similar to that which has been described the solid squares represent the etching with the revolvhereinbefore by referring to FIG. 2, except that the ing anode, while the hollow squares represent etching anode revolving speed was I00 rpm and 200 rpm. It with the stationary anode. It is apparent from FIG. 3 was found that similar improvement in the peak count that the bondage of the vitreous enamel layer with the and the PEI index can be achieved with the slower steel sheets etched as the revolving anode is generally revolving speed of the steel sheet, but the degree of the greater than that with steel sheets etched as the stationimprovements, e.g., the bonding strength of the enamel ary anodes. Similar trend is also apparent for the peak layer to the steel sheet, is enhanced with the increase of count. It should be noted that, when the peak count is the revolving speed. in excess of about 240, the PEI index of 100 percent In conventional pretreatment for applying a vitreous can be ensured without any failure. enamel layer in one process, it has been necessary to Table 7 Etching conditions Peak counts and FBI index Nickel plated Nickel plated Nickel plated Nickel plated with with with with Concentration Revolving Current 0.5 A/dm for 0.5 A/dm for 0.25 A/dm for 0.25 A/tlm for of sulfuric speed of conditions I00 seconds seconds 50 seconds 25 seconds acid drum (grams/liter) (rpm) (A/dmXsee) PEI PEI PEI PEI Peak" index Peak index Peak* index Peak index count (9%) count count ("7%) count ('14) 400 30 30 243 I00 240 100 245 I00 235 I00 400 30 30 220 an 220 I00 2|4 235 87.4

Number of peaks. whose height is greater than 0.5 micron, per 25.4 mm line section on steel sheet surface.

When the steel sheets are etched by using them as rotary anodes, their PEI indices are generally larger than those obtainable with steel sheets etched by using them as stationary anodes, even in the case that their peak count is relatively low.

FIGS. 4A, 4B, and 4C shows pictures of the surface conditions of a steel sheet which was etched by using it as a stationary anode, the pictures being taken by a scanning electron microscope at magnifications of L000, 3,000, and l0,000, respectively. FIGS. 5A, 5B,

reduce the weight of a steel sheet by about 0.2 g/dm by the etching. With the process according to the present invention, the weight loss in etching required for good adherence can be reduced to about one half or less of the conventional weight loss, by providing a comparatively high relative speed between the etching solution and the steel sheet which is to be etched thereby. Table 8 shows an example of such improvement in the weight reduction.

The reduced requirement for the weight reduction in the etching operation makes it possible to use a smaller electric power source and a shorter electrolytic cell in the actual etching equipment line.

In Example 10, the amount of the weight reduction by etching was controlled by varying the duration of the etching operation, while the nickel plating was carried out by using Watt bath with a current density of 0.5 A/dm for 30 seconds.

The inventors have found out through the experiments of Example that, to achieve good etched surfaces, a finite relative speed between the etching solution and the steel sheet is necessary, i.e., the etching solution at the steel sheet surfaces must be agitated.

Based on such finding, the inventors have worked out a practical device for carrying out the pretreating process for vitreous enamelling, of which an example is shown in FIGS. 6 and 7. In FIG. 6, rolled steel sheet 11 is fed to a pretreating process for vitreous enamelling by two pairs of guide rolls l2 and 13 in a direction as shown by the arrow 04. A pair of stationary cathode plates l4, 14' are disposed so as to face opposing surfaces of the steel sheet 11 with a spacing d from the corresponding steel sheet surface to the cathode 14 or 14'. The cathode plates l4, 14 are preferably protected by a pair of cover plates 15, 15'. Referring to FIGS. 6 and 7, a pair of nozzles l6, 16 are so located as to forcibly inject an etching solution at a high speed to the spacings between the steel sheet 11 and the cathodes l4, 14, from the opposite edges of sheet 11. Each nozzle I6 or 16' is connected to a pump 17 for delivering the etching fluid under pressure. The intake of each pump 17 communicates with a reservoir 18, which receives the etching fluid from the aforesaid spacing between the steel sheet 11 and the cathode plates l4, 14' for purposes of recirculation through the pump 17.

A pair of wiping rollers 19 engage the steel sheet 11 at opposite sides of the cathode plates 14, 14' to the nozzles l6, 16'. The wiping rollers l9, 19' are preferably made of acid-resisting rubber.

In the embodiment of FIGS. 6 and 7, an additional pair of anode plates 20, 20' are provided with suitable spacings d from the steel sheet 11. Pumps 22 deliver current-carrying fluid to the spacing d' under pressure through nozzles 21, 21', which nozzles direct the fluid in direction toward the first nozzles l6, 16'. The cathode plates 20, 20' are protected by covers 23, 23', and the fluid injected from the nozzles 21, 2] is wiped away by wiping rollers 24 before the sheet 11 faces the first cathode plates 14, 14'. The fluid from the spacings d is accumulated in a reservoir 25 for recirculation through the pumps 22, which reservoir 25 is separated from the first reservoir 18 by a partition wall 26.

The cathode plates l4, 14' are connected to a negative potential source (not shown), while the anode plates 20, 20' are connected to a positive potential source (not shown). Thus, a closed path is formed for a DC. current through the etching solutions and steel sheet 11.

It is apparent to those skilled in the art that, instead of the arrangement of FIGS. 6 and 7, the steel sheet 11 may be passed through a stationary etching solution with a suitable electric current feeding arrangement, so as to dispense with the nozzles 16, 16' and 21, 21'. To feed an electric current, electrically conductive rollers may be used which directly contact with the steel sheet 1 l.

With the embodiment of FIGS. 6 and 7, cold rolled low-carbon steel sheet of l meter width and 0.8 millimeter thickness was pretreated for vitreous enamelling by feeding it at 5 m/sec, while using an etching solution containing 2 percent sulfuric acid, which was recirculated at 10 m /min, and a current-carrying fluid containing 10 percent sulfuric acid, which was recirculated at 5 m /min. Each of the cathode plates 14, 14' was about 2 m long and spaced from the steel sheet 11 by about 10 cm, and the etching was effected with a DC. voltage of 20 volts and a current of 8,000 amperes.

FIGS. 8A, 8B, and 8C are pictures, showing the surface conditions of the steel sheet thus etched by the device of FIGS. 6 and 7, which pictures were taken by the same electron microscope as FIGS. 4A to 4C and 5A to SC with the corresponding magnifications, respectively. As can be seen from FIGS. 8A to BC, rectangular deep pits with stepped side walls can be formed by the process with the device of FIGS. 6 and 7, as in the case of FIGS. 5A to SC.

Thus, the steel sheet pretreated for vitreous enamelling by the process according to the present invention can strongly hold a vitreous enamel layer, which vitreous enamel layer may be applied thereto with or without nickel plating and shaping of the steel sheet prior to the enamelling. Upon baking, the vitreous enamel layer applied to the steel sheet, which is pretreated by the process of the present invention, is bonded to the steel sheet much more steadfastly than to conventionally pretreated steel sheets.

As described in the foregoing disclosure, according to the present invention, there is provided an improved pretreating process for vitreous enamelling which may include electrolytic etching with a liquid electrolyte moving relative to a steel sheet to be etched, whereby deep etching pits are formed for greatly strengthening the bondage between the steel sheet and the vitreous enamel layer applied thereto. The etching solution may forcibly be injected under pressure to the steel sheet surface in jet streams by nozzles, so as to make it possible to use electrolyte cells with reduced lengths. Thus, the vitreous enamelling can be carried out by a compactly built facilities. Accordingly, the invention contributes greatly to the industry.

What is claimed is:

l. A vitreous enamelling process for a steel sheet, comprising roughening the steel sheet by electrolysis, applying an aqueous solution of water glass on the roughened surface and drying the solution to form a water glass layer thereon, press shaping the steel sheet having the water glass layer thereon, washing away the water glass layer with water and applying a vitreous enamel layer on the roughened surface of the steel sheet after the water glass layer is washed away.

2. A process according to claim 1 and further comprising a step of applying a solution of a wax dissolved in an organic solvent on the water glass layer so as to form a wax coating on the water glass layer, said wax coating normally being solid phase at room temperature.

3. A vitreous enamelling process to claim 1, wherein said roughening is effected by positioning a cathode plate parallel and in close proximity to the surface of the steel sheet so that a gap is formed therebetween, injecting a liquid electrolyte under pressure and at high speed into the gap between the cathode plate and the steel sheet so that the electrolyte fills the gap while 17 quickly passing therethrough, and applying a potential across the cathode plate and the steel sheet to cause the steel sheet to act as an anode and etch the steel sheet surface by a current flowing between he cathode plate and the steel sheet.

4. A vitreous enamelling process according to claim 3, wherein the steel sheet is continuously moved at high speed relative to the cathode plate during the etching operation, and said liquid electrolyte is recirculated through a closed loop system including a reservoir, a pressurizing pump, a nozzle means, and said gap be tween the cathode plate and the steel sheet, said liquid electrolyte being delivered to the nozzle means under pressure by said pressurizing pump so that the nozzle means injects the liquid electrolyte into the gap between the cathode plate and the steel sheet under pressure and at high speed.

5. A process according to claim 1, wherein said roughened surface of the steel sheet is phosphate coated before being provided with said water glass layer.

6. A process according to claim 1, wherein said roughened surface of the steel sheet is nickel plated before being provided with said water glass layer.

7. A process according to claim 1, wherein said steel sheet is continuously moved during said toughening operation relative to a stationary cathode plate positioned parallel and in close proximity thereto to form a gap therebetween, a liquid electrolyte under pressure is injected at high speed into the gap between the steel sheet and the cathode plate so that the electrolyte fills the gap while quickly passing therethrough, an anode plate is positioned parallel and in close proximity to the steel sheet to form a gap therebetween at a point different from said cathode plate, an electrically conductive liquid under pressure is injected at high speed into the gap between the anode plate and the steel sheet so that the electrolyte fills the gap while quickly passing there through, and applying a potential across the anode plate and the cathode plate thereby etching the surface of the steel sheet by an electric current flowing between the anode and cathode plates through the steel sheet.

8. A vitreous enamelling process according to claim 1, wherein the water contains an active amount of an alkaline compound selected from the group consisting of sodium hydroxide, sodium silicate, and sodium phos phate.

9. A vitreous enamelling process according to claim 8, wherein the water contains a surface active agent.

10. A vitreous enamelling process according to claim 1, wherein said water glass layer is about 2 to 5 micron thick. 

1. A VITREOUS ENAMELLING PROCESS FOR A STEEL SHEET, COMPRISING ROUGHENING THE STELL SHEET BY ELECTROLYSIS, APPLYING AN AQUEOUS SOLUTION OF WATER GLASS ON THE ROUGHENED SURFACE AND DRYING THE SOLUTION TO FORM A WATER GLASS LAYER THEREON, PRESS SHAPING THE STEEL SHEET HAVING THE WATER GLASS LAYER THEREON,PRES WASHING AWAY THE WATER GLASS LAYER WITH WATER AND APPLYING A VITREOUS ENAMEL LAYER ON THE ROUGHENED SURFACE OF THE STEEL SHEET AFTER THE WATER GLASS LAYER IS WASHED AWAY.
 2. A process according to claim 1 and further comprising a step of applying a solution of a wax dissolved in an organic solvent on the water glass layer so as to form a wax coating on the water glass layer, said wax coating normally being solid phase at room temperature.
 3. A vitreous enamelling process to claim 1, wherein said roughening is effected by positioning a cathode plate parallel and in close proximity to the surface of the steel sheet so that a gap is formed therebetween, injecting a liquid electrolyte under pressure and at high speed into the gap between the cathode plate and the steel sheet so that the electrolyte fills the gap while quickly passing therethrough, and applying a potential across the cathode plate and the steel sheet to cause the steel sheet to act as an anode and etch the steel sheet surface by a current flowing between he cathode plate and the steel sheet.
 4. A vitreous enamelling process according to claim 3, wherein the steel sheet is continuously moved at high speed relative to the cathode plate during the etching operation, and said liquid electrolyte is recirculated through a closed loop system including a reservoir, a pressurizing pump, a nozzle means, and said gap between the cathode plate and the steel sheet, said liquid electrolyte being delivered to the nozzle means under pressure by said pressurizing pump so that the nozzle means injects the liquid electrolyte into the gap between the cathode plate and the steel sheet under pressure and at high speed.
 5. A process according to claim 1, wherein said roughened surface of the steel sheet is phosphate coated before being provided with said water glass layer.
 6. A process according to claim 1, wherein said roughened surface of the steel sheet is nickel plated before being provided with said water glass layer.
 7. A process according to claim 1, wherein said steel sheet is continuously moved during said roughening operation relative to a stationary cathode plate positioned parallel and in close proximity thereto to form a gap therebetween, a liquid electrolyte under pressure is injected at high speed into the gap between the steel sheet and the cathode plate so that the electrolyte fills the gap while quickly passing therethrough, an anode plate is positioned parallel and in close proximity to the steel sheet to form a gap therebetween at a point different from said cathode plate, an electrically conductive liquid under pressure is injected at high speed into the gap between the anode plate and the steel sheet so that the electrolyte fills the gap while quickly passing therethrough, and applying a potential across the anode plate and the cathode plate thereby etching the surface of the steel sheet by an electric current flowing between the anode and cathode plates through the steel sheet.
 8. A vitreous enamelling process according to claim 1, wherein the water contains an active amount of an alkaline compound selected From the group consisting of sodium hydroxide, sodium silicate, and sodium phosphate.
 9. A vitreous enamelling process according to claim 8, wherein the water contains a surface active agent.
 10. A vitreous enamelling process according to claim 1, wherein said water glass layer is about 2 to 5 micron thick. 